Written feedback provided by the teacher to his or her students is an important aspect of formative assessment. After a theoretical introduction to teacher prerequisites for giving feedback and to the quality of written feedback in general, results from an implementation of feedback methods in classrooms will be described for the cases of Germany, Switzerland and Denmark. The focus will be on the inquiry method ‘investigation in science’ that requires from students such competences as planning and/or conducting experiments. This study examines the quality of written teacher feedback which was provided based on rubrics and templates for open comments. For this purpose, written teacher feedback itself, student artefacts and data from questionnaires were analysed. Furthermore, the benefits and challenges that teachers noticed in using written feedback will be examined. Finally, it will be discussed which means of support for teachers seem necessary in order to foster the implementation of written teacher feedback as part of formative assessment in inquiry-based science education.